Participants in a networked environment sometimes need to communicate commands or announcements to other participants. Participants are applications, users, and generally any entity that uses services provided by the networked environment. Examples of commands include “restart the operating system,” “close the application,” and so forth. Examples of announcements include “service is terminating,” global positioning system coordinates of a user, team meetings, and so forth. These commands or announcements can be referred to as alerts. The source and target of these alerts are endpoints, which are devices or software that represent a participant in the networked environment. Each participant can have multiple endpoints. As an example, a user or application can use a mobile computer as an endpoint as well as a desktop computer as an endpoint. Some or all endpoints associated with a participant may need to receive alerts.
The endpoints may employ various mechanisms to communicate messages to one another, such as a Session Initiation Protocol (“SIP”). SIP is an application-layer control protocol that endpoints can use to discover one another and to establish, modify, and terminate communications sessions. SIP is an Internet-proposed standard. Its specification, “RFC 3261,” is available at the Internet Engineering Task Force's web site (www.ieff.org). A specification for extensions to SIP relating to event notifications, “RFC 3265,” is also available at the web site. Both of these specifications are incorporated herein in their entirety by reference.
An exchange of messages during a SIP session is generally referred to as a dialog. SIP may use lower-level communications layers to transport a dialog's messages, such as Transmission Control Protocol/Internet Protocol (“TCP/IP”), which are commonly employed transport- and network-layer protocols. SIP messages can also be sent outside dialogs. This is sometimes referred to as Page Mode. In Page Mode, a sender sends a SIP message to a recipient without any expectation of a session being established. SIP Page Mode messages are generally characterized by their lack of Contact header fields. SIP supports multiple message types, including MESSAGE and NOTIFY. Initial messages (whether MESSAGE or NOTIFY) that are sent before a session or dialog is established are referred to as requests.